In an earlier work, Wing et al. [1] reported on the creep response
of polycarbonate (PC) and microcellular polycarbonate (MCPC). MCPC is a
novel cellular material with cells on the order of 10 [micro]m and a
cell density on the order of [10.sup.10] cells per [cm.sup.3]. Kumar and
Weller [2] describe a two-stage process to produce MCPC. The first stage
consists of saturating the polymer with C[O.sub.2] gas in a pressure
vessel at room temperature. In the second stage, the gas-saturated
polymer is removed from the pressure vessel and heated above its glass
transition temperature. Bubble nucleation and growth ensue in the
polymer upon heating, and the cells are limited in size by the high
viscosity of the polymer near the glass transition temperature. At the
appropriate time, the foamed polymer is quenched in room temperature
water to prevent further bubble growth. The gas in the cells and in the
polymer matrix eventually diffuses out to the atmosphere. This class of
cellular polymers has come to be known as solid-state foams, as the
process temperatures are in the vicinity of the glass transition
temperature of the polymer. A brief review of solid-state microcellular
foams can be found in Kumar [3].

The motivation for studying the effect of C[O.sub.2] sorption and
desorption on the creep of PC is in the context of understanding the
creep behavior of MCPC. In the process of producing MCPC, the polymer is
subjected to a cycle of C[O.sub.2] sorption and desorption. It has been
recognized by Seeler and Kumar [4] and Kumar et al. [5] that the gas
sorption phase of the microcellular process may affect the mechanical
properties of the polymer foam matrix. Therefore, it is expected that
the creep response of MCPC will depend on both the foam microstructure as well as on the effect of the cycle of gas sorption and desorption on
the PC matrix.

It has been well documented by Crissman [6], Crook and Letton [7],
Nishitani et al. [8], Peretz and Weitsman [9, 10], and Zapas and
Crissman [11] that amorphous polymers exhibit time-dependent response to
an applied load; Yee et al. [12] showed that this response becomes more
vigorous with an increase in temperature. Also, when polymers are cooled
to below their glass transition temperature, the glass formation process
begins. Immediately after cooling below the glass transition
temperature, the material is in a nonequilibrium state and its free
volume continues to evolve toward equilibrium. Changes in the mechanical
(viscoelastic) behavior associated with changes in the glassy structure
have come to be known as "physical aging" [13].

Several researchers have studied the effect of nonreacting gases on
the mechanical behavior of polymers. Seeler and Kumar [4] investigated
the effect of C[O.sub.2] saturation and desorption on the fatigue life
of PC. They reported that the fatigue life of the saturation-cycled PC
exceeded that of unprocessed PC by up to a factor of 30. The increase in
fatigue life was found to be a function of the C[O.sub.2] saturation
pressure. Wissinger and Paulaitis [14] have reported that the glass
transition temperature, [T.sub.g], of polymethyl methacrylate (PMMA) and
polystyrene (PS) are reduced by C[O.sub.2] sorption. Change in modulus
of silicone elastomer as a function of nitrogen gas pressure has been
studied by Briscoe and Zakaria [15]. Chan and Paul [16] found that a
cycle of gas sorption and desorption has lasting effects on the gas
sorption, and thermal and static mechanical properties of glassy
polymers. They attributed the effects to the increase in free volume of
the polymer that results from a saturation cycle. Hojo and Findley [17]
reported increased creep in PC tubes pressurized with C[O.sub.2]. In an
article on the effects of physical aging and C[O.sub.2] absorption in
bisphenol-A-PC, Risch and Wilkes [18] found that absorbed C[O.sub.2]
dramatically reduced the glass transition temperature of PC with linear
dependence on absorbed mass fraction.

EXPERIMENTAL

Specimen Preparation

ASTM type IV tensile specimens were chosen for use in this study.
The specimens were machined to the specifications of ASTM D638. Type IV
tensile specimens have a specified gage length of 25 mm and a throat
width of 6 mm. The specimens were machined from 1.57-mm-thick sheets of
LEXAN 9034 PC with a density of 1.20 g/[cm.sup.3]. Holes were drilled in
the center of both grip portions of the specimens at the time of
manufacture. The holes were drilled to locate the self-aligning pins in
the grips used to hold the specimens.

Polymer sheets are commonly produced by extrusion, resulting in
some alignment of molecular chains in the polymer matrix and anisotropy in the physical properties of the polymer. All of the specimens used in
this study were machined from the PC sheets with the same relative
orientation to eliminate variations due to extrusion-induced anisotropy.
Residual stresses in tensile specimens disturb the otherwise uniform
uniaxial stress field produced by uniaxial loading. The specimens were
inspected under a polariscope to determine the amount of
machining-induced residual stress. Based on this inspection, it was
found that the machining did not develop significant residual stresses.

The tensile specimens used to study the effects of exposure to
C[O.sub.2] gas on the creep behavior of PC were placed in a pressure
vessel and exposed to a C[O.sub.2] environment for 60 hours at various
pressures at room temperatures. Table 1 lists the parameters of the gas
sorption process used to produce these tensile specimens. Observation of
specimens under a polariscope revealed no significant residual stresses
in any of the specimens due to C[O.sub.2] saturation. These saturated
specimens were tested immediately after removal from the pressure
vessel.

Creep Test Equipment and Procedure

The creep/creep-recovery tests were performed using dead-weight
tensile loading. An electric weight trolley car was employed to smoothly
lower and raise the weights. Self-aligning grips were employed to load
the specimens. Strains in the tensile specimens were measured using an
MTS 632.11b strain gage based contacting extensometer. The strains were
recorded using a computer-controlled data acquisition system. The data
acquisition system was capable of recording strains as high as 0.374
with a resolution of 0.0015. Strain data was collected at various time
intervals over the entire creep/creep-recovery test period. The
creep/creep-recovery tests consisted of an 8-hour creep period, followed
by a 2-hour creep-recovery period. All tests were performed at ambient
room temperature and conditions.

Creep/Creep-Recovery Tests

The first series of creep/creep-recovery tests were performed on
"as-received" polycarbonate specimens that were not exposed to
C[O.sub.2] gas. These tests were conducted to characterize the
viscoelastic and viscoplastic response of the LEXAN 9034 PC prior to
testing the "saturated" PC. The as-received PC tensile
specimens were tested at eight stress levels ranging from 13.8 to 51.7
MPa. At least three tests were performed at every stress level tested.

PC tensile specimens saturated at 5.5 MPa were subjected to
creep/creep-recovery tests at stress levels ranging from 13.8 to 34.5
MPa to evaluate the effect of C[O.sub.2] saturation on the creep
response. In addition, creep/creep-recovery tests were performed on
specimens saturated at pressures ranging from 2.1 to 5.5 MPa, as listed
in Table 1, to determine the effect of saturation pressure on the creep
behavior. These creep/creep-recovery tests were performed at a stress
level of 20.7 MPa.

The effect of C[O.sub.2] desorption on the creep response was
investigated by testing specimens that had been saturated and then
desorbed for times ranging from 6 to 60 days. The creep/creep-recovery
experiments at various desorption periods were conducted at a stress of
20.7 MPa.

A separate series of creep/creep-recovery tests were conducted on
specimens that had been initially saturated at 5.5 MPa and then allowed
to desorb for 60 days. These specimens were tested at stress levels of
13.8, 20.7, and 34.5 MPa.

[FIGURE 1 OMITTED]

Desorption Experiments

To determine the desorption rate of C[O.sub.2] gas from the PC
matrix, several specimens were first saturated in C[O.sub.2] gas at 5.5
MPa for a period of 60 hours. These specimens were then removed from the
pressure vessel and the desorption rate of the C[O.sub.2] gas from the
PC matrix was determined by weight loss. Weights were measured using a
Mettler AE240 balance with a resolution of 10 [micro]g. Desorption data
was collected until the amount of C[O.sub.2] remaining in the PC
specimens was approximately 1% of the amount at saturation.

RESULTS

Creep Behavior of PC

No appreciable creep was observed in the as-received PC within the
resolution of the data acquisition system, at stress levels of 13.8 MPa
or below. An isochronous stress-strain curve using strain data obtained
at 1 minute and 470 minutes from the initial loading time is shown in
Fig. 1. From this data, the creep response of PC was found to be
nonlinear at stress levels above 24 MPa. Creep and recovery data at
several stress levels are shown in Fig. 2. At the stress level of 51.7
MPa the creep strains induced in the PC at the end of 8 hours approached
6%.

[FIGURE 2 OMITTED]

Creep Behavior of Saturated PC

Figure 3 shows the creep response of saturated and as-received PC
at stress levels of 20.7 and 34.5 MPa. At the 20.7 MPa stress level
there is a small increase in the creep strains of the saturated
specimens. However, at the 34.5 MPa stress level the effect of the
dissolved gas on the creep is significant. The exposure to C[O.sub.2]
has a pronounced effect on both the time-dependent response of the
material as well as its instantaneous response. From the instantaneous
response at the 34.5 MPa stress level in Fig. 3, the tensile modulus of
as-received PC is estimated to be 2.6 GPa, while the tensile modulus of
saturated PC is approximately 1.7 GPa, a drop of some 35%. This
magnitude of drop in the modulus is analogous to the behavior we might
expect upon heating the polymer to a temperature near the glass
transition temperature of PC.

PC tensile specimens were saturated with C[O.sub.2] gas at six
saturation pressures ranging from 2.1 to 5.5 MPa for a period of 60
hours. These specimens were then subjected to creep/creep-recovery tests
at a stress of 20.7 MPa. The results of these tests are shown in Fig. 4.
We see that the creep response of PC is insensitive to saturation
pressure in the range from 2.1 to 5.5 MPa. Thus, increasing the
concentration of the dissolved C[O.sub.2] beyond the 4.54% at 2.1 MPa
(see Table 1) does not appear to further influence the creep response at
this stress level.

[FIGURE 3 OMITTED]

Desorption of C[O.sub.2] from Saturated PC

Figure 5 shows the result from the desorption experiment. Here, we
have plotted the percent of gas remaining in a 1.5-mm-thick PC specimen
saturated at 5.5 MPa and then left at atmospheric pressure to desorb at
room temperature, as a function of time. It can be seen that almost 70%
of C[O.sub.2] leaves the specimen within one day and the specimens have
less than 1% of gas remaining after approximately 11 days. This data is
in agreement with that of Seeler and Kumar [19] who report that it takes
another 8 days for the last 1% of gas to leave the specimen.

Creep Behavior of Saturation-Cycled PC

Figure 6 shows data from the creep/creep-recovery tests performed
on saturation-cycled PC specimens with desorption times ranging from 6
to 60 days at a stress of 20.7 MPa. Several observations can be made
from this data. First, we see that as the desorption time increases the
creep response of the saturation-cycled PC appears to approach that of
the as-received PC. Second, we observe that the creep compliance of PC
specimens continues to vary over the 60 days of desorption although, as
seen from Fig. 5, only approximately 1% of C[O.sub.2] remains in the PC
after 11 days of desorption, and no measurable amount of gas remains
after 19 days of desorption [19].

[FIGURE 4 OMITTED]

Figure 7 shows creep/creep recovery data for specimens saturated at
5.5 MPa and then allowed to desorb for 60 days at stress levels ranging
from 13.8 to 34.5 MPa. The behavior is compared to as-received PC at the
same stress levels. For saturation-cycled PC the creep behavior is seen
be linear at stress levels of 20.7 MPa and lower. At low stress levels
the creep response of the saturation-cycled PC is similar to that of
as-received PC. However, at a higher stress level of 34.5 MPa, the
saturation-cycled PC shows considerably more creep than the as-received
PC. Thus, the effect of C[O.sub.2] saturation on PC creep behavior
persists even after 60 days from the time of saturation.

[FIGURE 5 OMITTED]

DISCUSSION

It is clear from the data presented that when the PC is saturated
with C[O.sub.2] at 2.1 MPa (corresponding to a gas concentration of
4.54%) or a higher pressure, the creep initiates at a lower stress and
the PC becomes more compliant. These observations are consistent with
the finding that the dissolved gas lowers the glass transition
temperature of the polymer by increasing the molecular mobility [2, 14,
18]. This effect on the glass transition can be quite significant. For
example, for PC saturated with C[O.sub.2] at 4.8 MPa, Kumar and Weller
[2] estimate that the glass transition temperature lowers from
150[degrees]C for virgin or as-received PC to 87[degrees]C, a change of
63[degrees]C. Interestingly, increasing the concentration of dissolved
C[O.sub.2] beyond 4.54% by using a higher saturation pressure does not
lead to a further increase in PC creep response (Fig. 4).

[FIGURE 6 OMITTED]

The tests on saturation-cycled specimens showed that after PC is
saturated with C[O.sub.2], and is then allowed to desorb at atmospheric
pressure, the creep compliance continues to change with time. After 11
days of desorption, the PC compliance is still significantly higher than
the as-received PC (Fig. 6). This trend continues for specimens desorbed
(or "aged") for 60 days after saturation (Fig. 7), indicating
that the effect of C[O.sub.2] saturation-cycling is long-lasting and
could be permanent. This observation is consistent with Seeler and Kumar
[19] who report that the yield strength of saturation-cycled PC tested
after 62 days of desorption was 26% lower than the as-received PC. Since
the amount of gas dissolved in the specimen drops to about 1% of the
concentration after 11 days (Fig. 5) and only trace amounts of gas
remain in the matrix after this time, the persistent change in creep
compliance of PC can not be explained on the basis of enhanced molecular
mobility caused by the dissolved gas. It is suggested that saturating PC
with C[O.sub.2] increases free volume and the material structure
continues to evolve during desorption of C[O.sub.2]. After all the gas
has left, which in this study takes about 20 days, the properties still
change with time, showing continued physical aging. These findings
parallel those of Risch and Wilkes [18] who found that PC samples that
had been aged after absorbing a mass fraction of 0.07-0.10 of C[O.sub.2]
showed thermal and mechanical behavior similar to that of polymer
quenched from above [T.sub.g] with identical absorbed mass fraction of
gas.

SUMMARY

The effect of C[O.sub.2] sorption and desorption on the creep
response of PC was studied. Tensile specimens machined from PC sheets
were exposed to C[O.sub.2] and the absorbed gas mass fraction ranged
from 0.045 to 0.12. The creep/creep recovery response of as-received PC,
saturated PC, and saturation-cycled PC was characterized. It was found
that the saturated PC showed a creep behavior similar to heating the PC
to its glass transition temperature. The creep compliance of
saturation-cycled PC was found to change with the desorption or aging
time. The tests on PC saturated and then desorbed for up to 60 days
showed that the effects of exposure to C[O.sub.2] on PC creep properties
persist long after the gas has left the polymer, and could be permanent.

[FIGURE 7 OMITTED]

ACKNOWLEDGMENTS

We thank the member companies of the University of Washington,
Industry Cellular Composites Consortium for their support. Special
thanks to graduate student Sravani Pakala for preparing the figures.